It is known that sulfur dioxide (SO2) contained in exhaust from an internal combustion engine becomes sulfur trioxide (SO3) in an oxidation catalyst and then becomes H2SO4 through a reaction with water (H2O). H2SO4 is turned into white smoke (sulfate white smoke) and discharged to the atmosphere in some cases. This chemical reaction is disclosed in PTL 1. In PTL 1, it is also disclosed that SO3 becomes more likely to be generated as the amount of surplus oxygen in the oxidation catalyst increases when an exhaust gas temperature becomes equal to or higher than a predetermined temperature and the surplus oxygen is decreased so as to avoid a rapid increase in the amount of SO3 sulfuric acid mist attributable thereto.